The present invention relates to a composition that is useful to protect and promote adhesion to a metallic substrate, particularly aluminum.
Currently, there are numerous commercial methods for treating metal substrates to prepare them for application of overcoats. These methods are occasionally referred to as conversion treatments. One such method is known as the ALODINE method available from Parker Amchem. According to this method, the substrate is etched with phosphoric acid and then rinsed. A conversion composition then is prepared from a concentrate that includes hexavalent chrome. This conversion composition then is flowed over the etched surface. Another method is a 9 to 13 step method that involves a manganese conversion coating that includes the application of heat.
Consequently, it would be very advantageous to provide a chrome-free, low volatile organic, shelf stable protective and adhesion promoting composition for a metal substrate. It would also be advantageous if this composition could be applied to the substrate via a single pack system that requires only a few steps.
Coating compositions that are made from or include in general a silane, water and phosphoric acid are known (see U.S. Pat. No. 4,098,749; U.S. Pat. No. 5,091,469; and DE-A-4403876).
DE-A-4403876 discloses a composition that is said to form a passivating layer that is a polysiloxane structure that includes phosphorus-oxygen links (Si--O--Si--O--P--O--Si) when coated on a metal substrate. In order to achieve this structure, it is alleged that "the percentage of phosphoric acid must be higher than required for mere catalysis." However, the single example includes only approximately 1 wt % phosphoric acid. In addition, an organic solvent (ethylene glycol monomethyl ether) and an alcohol (methanol or 2-propanol) are used as initial components to make the coating composition. This application also indicates that the coating bath is maintained at a pH of &lt;7, preferably a pH of 5 and 6.
Claim 1 of U.S. Pat. No. 4,098,749 claims an anticorrosion primer coating composition comprising 100 parts by weight (pbw) polyvinyl butyral resin, 1-25 pbw of at least one organofunctional silane, 20-35 pbw of an inorganic borate and/or polyphosphate compound, and 5 to 50 pbw phosphoric acid. Upon close inspection of the examples in the U.S. Pat. No. 4,098,749 patent it is clear that the pbw ranges recited in claim 1 do not take into account the weight of any carriers such as an organic solvent or water.
The largest amount of phosphoric acid in any of the examples appears to be in Example 13. 18 pbw of an 85% aqueous solution of phosphoric acid is mixed with 16 pbw water and 66 pbw isopropyl alcohol to form an "additive" composition. 20 parts of this "additive" is mixed with 80 parts of a main composition that includes the polyvinyl butyral and the silane. Based on this description, the calculated amount of phosphoric acid in the final coating composition is 3.6 wt %. It is also clear from the examples that the U.S. Pat. No. 4,098,749 system is a two pack system and that it includes an organic solvent.